add a link to composer-dev mailing list to /about-composer

src/Packagist/WebBundle/Resources/views/About/aboutComposer.html.twig

@@ -120,5 +120,6 @@
 
         <h1>Community</h1>
         <p>If you have questions about composer or want to help out, come and join us in the <em><a href="irc://irc.freenode.net/#composer-dev">#composer-dev</a></em> channel on irc.freenode.net.</p>
+        <p>Also join the <a href="http://groups.google.com/group/composer-dev">composer-dev mailing list</a>.</p>
     </div>
 {% endblock %}